Basic Operations on Graphs Lecture 5.

Basic Operations on Graphs Deletion of edges Deletion of vertices Addition of edges Union Complement Join

Deletion of Edges If G = (V,E) is a graph and e 2 E one of tis edges, then G - e := (V,E – {e}) is a subgraph of G. In such a case we say that G-e is obtained from G by deletion of edge e.

Deletion of Vertices Let x 2 V(G) be a vertex of graph G, then G - x is the subgraph obtained from G by removal of x grom V(G) and removal of all edges from E(G) having x as an endpoint. G – x is obtained from G by deletion of vertex x.

Exercises 01 N1. Show that for any set F µ E(G) the graph G-F is well-defined. N2. Show that for any set X µ V(G) the graph G- X is well-defined. N3. Show that for any set X µ V(G) and any set F µ E(G) the graph G-X-F is well-defined. N4. Prove that H is a subgraph of G if and only if H is obtained from G by a succession of vertex and edge deletion.

Edge Addition Let G be a graph and (u,v) a pair of non- adjacent vertices. Let e = uv denot the new edge between u and v. By G’ = G + uv = G + e we denote the graph obtained from G by addition of edge e. In other words: V(G’) : = V(G), E(G’) : = E(G) [ {e}.

Graph Union Revisited If G and H are graphs we denote by G t H their disjoint union. Instead of G t G we write 2G. Generalization to nG, for an arbitrary positive integer n: –0G := ;. –(n+1)G := nG t G Example: Graph in top row C 6 t K 9 Graph in bottom row 2K 3.

Graph Complement Graph complement G c of simple graph G has V(G c ) := V(G), but two vertices u in v are adjacent in G c if and only if they are not adjacent in G. For instance C 4 c is isomorphic to 2K 2.

Graph Difference If H is a spanning subgraph of G we may define graph difference G \H as follows: V(G\H) := V(G). E(G\H) := E(G)\E(H). G HG\H

Bipartite Complement For a bipartite graph X (with a given biparitition) one can define a bipartite complement X b. This is the graph difference of K m,n and X: X b = K m,n \ X. X XbXb

Empty Graph Revisited. The word “empty graph” is used in two meanings. First Meaning: ;. No vertices, no edges. Second Meaning: E n := K n c.= nK 1. There are n vertices, no edges. E 0 = ; = 0. G will be called the void graph or zero graph.

Graph Join Join of graphs G and H is denoted by G*H and defined as follows: G*H := (G c t H c ) c In particular, this means that K m,n is a join of two empty graphs E n and E m.